Photovoltaic solar cells are thin silicon disks that can be used to convert sunlight into electricity and serve as an energy source for a wide variety of uses. For example, small area solar cells can be used to power calculators, cell phones and other small electronic devices. Larger panels can be used for supplementing or fulfilling the electrical needs of individual residences, lights, pumping, cooling, heating, etc.
Research into the use of solar energy as a power source began as early as 1839 with the discovery that materials that were sensitive to light could be used to convert sunlight into electricity. An early solar cell, made of gold-coated selenium was produced in 1877, although it was only one percent efficient, i.e. converted only one percent of the incoming sunlight into electricity. Einstein's explanation of the photoelectric effect in 1905 spurred new interest in producing solar electricity at higher efficiencies. However, little progress was made until advances in diodes and transistors allowed silicon solar cells exhibiting four percent efficiency to be produced in 1954. Further work produced solar cells having efficiency up to 15 percent that were used in rural and isolated areas as power sources for a telephone relay system.
In order to meet domestic energy needs, efficiency of solar cells had to be further improved, while maintaining cost effectiveness. Conventional silicon based high efficiency solar cells are produced from single crystalline silicon. In order to make single crystalline silicon wafers, pure silicon starting material must first be obtained. Pure silicon is produced from silicon dioxide of either quartzite gravel or crushed quartz that has been processed in an electric arc furnace to release oxygen and produce carbon dioxide and molten silicon. While this process yields silicon with only one percent impurities, the solar cell industry requires higher purity. One way to produce high purity silicon is to further process the 99 percent pure silicon using a floating zone technique wherein a rod of silicon is passed through a heated zone several times in the same direction. This procedure acts to drag the impurities toward one end of the silicon rod. Once the silicon reaches the desired purity level, the end containing the impurities is removed.
Another method for producing high purity single crystalline wafers is known as the Czochralski method. In this process, a boule of silicon is created, by repeatedly dipping a seed crystal of silicon into melted silicon. As the seed crystal is withdrawn and rotated, a cylindrical ingot, the boule, of single crystalline silicon is formed. This boule is highly pure because impurities tend to remain in the liquid silicon.
Silicon wafers are sliced from the boule one at a time using a single blade circular saw or many at time using a multiwire saw. Slicing results in loss of up to half of the original boule and further cutting may be carried out to shape the wafers into rectangles or hexagons, for fitting together into a solar cell array. The slicing and cutting of the wafers creates roughness and defects caused by saw-damage. These areas of roughness and damage must be removed in order to form an abrupt, defect free p-n junction and contact wires needed for the final solar cell. Roughness and damage is typically removed by an aggressive anisotropic etch process known as “damage etch”.
Several different etch solutions have been proposed and used to perform the damage etch. The most common technique for single crystals uses an etching solution of KOH or NaOH solutions in deionized (DI) water at about 80° C. However, the use of these etching solutions exhibit significant disadvantages. In particular, KOH or NaOH solutions do not wet the silicon surface well, and frequently experience non-uniform hydrogen bubble buildup that prevents uniform contact between the silicon surface and the etching solution. This can result in non-uniform etching across the wafer, which leads to variation in solar cell efficiency and lower reproducibility of the solar cell product. In addition, the KOH or NaOH solutions do not efficiently or effectively remove contaminants that are caused by the saw slicing and do not passivate the surface of the silicon, again resulting in a reduction in the efficiency of the solar cells produced.
Other solutions have been used to etch silicon, but have not been suggested for use in solar cell damage etch processes. Rather, tetramethylammonium hydroxide (TMAH), isopropyl alcohol (IPA) and pyrazine have been used to etch silicon for use in MEMS applications. These solutions provide an etch that obtains a flat surface with minimal undercutting of mask layers. (see Chung, Anisotropic Etching Characteristics of Si in Tetramethylammonium Hydroxide: Isopropyl Alchohol: Pyrazine Solutions, Journal of the Korean Physical Society, Vol. 46, No. 5, May 2005, pp. 1152-1156).
The result of the damage etch process is a silicon wafer that is very shiny and reflective. The efficiency of a solar cell is determined by the ability to gather or absorb light. While silicon has a large absorption coefficient in the visible light spectrum, it also exhibits a high reflection coefficient. To increase efficiency of solar cells, the reflectivity of the damage etched silicon wafer must be reduced. One common method of reducing reflectance is to coat the silicon wafer with an anti-reflective coating (ARC), such as silicon oxide, silicon nitride or titanium dioxide. However, these films exhibit resonance structures that limit their effectiveness to a small range of angles and wavelengths, such that efficiency depends on the angle of incidence of the light.
Another method of reducing reflectance and improving solar cell efficiency is to texture the silicon wafer surface using a wet-chemical etch to from pyramidal structures. These structures provide higher levels of light trapping based on geometrical optics, e.g. the texturing is on a scale equal to or greater than optical wavelengths of the incident light causing the incident light to reflect multiple times and thereby enhance absorption.
The texturing process is generally carried out using a mixture of KOH or NaOH and PA in DI water as the etchant. (Sec U.S. Pat. No. 3,998,659; Gangopadhyah et al, A novel low cost texturization method for large area commercial mono-crystalline silicon solar cells, Solar Energy Materials & Solar Cells, 90, 2006, pp. 3557-3567; King et al., Proceedings of 22nd IEEE International Photovoltaic Specialists Conference, Las Vegas, 1991, pp. 303-308). The addition of IPA serves to mask specific silicon sites, preventing etching by the solution, to thereby form the pyramidal structures. It has also been reported that a combination of IPA and aqueous alkaline ethylene glycol resulted in more uniform pyramidal texturing on highly polished silicon (100) for use in semiconductor electronic applications. (See U.S. Pat. No. 6,451,218). In addition, the use of sodium acetate, anhydrous (CH3COONa) operates in a similar manner to IPA for alkaline texturing, however the two compounds can not co-exist. (see Zhenqiang Xi et al, Investigation of texturization for monocrystalline silicon solar cells with different kinds of alkaline, Renewable Energy 29, 2004, pp. 2101-2107). In none of the references noted above has the use of the solutions mentioned been applied to as-cut silicon wafers for purposes texturing a sample that still has saw damage and contamination.
As noted above, the diluted KOH or NaOH damage etch solutions have poor silicon wetability characteristics and do not remove surface contamination effectively, leading to reduced solar cell efficiency and reproducibility.
There remains a need in the art for improvements to the efficiency of solar cells and to methods of performing damage etch and texturing of single crystal silicon substrates.